Steel plates for use in marine structures, construction machines, bridges, pressure vessels, storage tanks, buildings, and the like are required to have high toughness from the viewpoint of safety in addition to high yield strength and high tensile strength.
In general, in order to achieve high strength and high toughness in steel microstructures, the refinement of grains is known to be effective. For example, Patent Literatures 1 to 8 disclose a method for increasing the toughness of a steel plate or sheet by microstructural refinement.